Arc plasma spraying is a method for depositing materials on various substrates. A DC electric arc creates an ionized gas (a plasma) which is used to literally spray molten powdered materials in a manner similar to spraying paint. It was first developed by the aerospace industry where a need existed for different high performance coatings that afforded better thermal protection, electrical insulation and mechanical wear resistance. The technology has since found many applications in other industries because the coatings are typically of high density, good adhesion, and relatively low cost. One of the major advantages of the method is that it is amenable to volume manufacturing. For example, plasma sprays have been used to manufacture millions of alternator parts per year with aluminum oxide.
Plasma spray is part of a larger class of technology called, thermal spray, which includes combustion and electronic arc methods for depositing metal and ceramic coatings.
A thermal sprayed coating has a unique microstructure. During the deposition process, each particle enters a gas stream, melts, and cools to the solid form independent of other particles. When molten particles impact the substrate being coated, they impact (xe2x80x9csplatxe2x80x9d) as flattened circular platelets and freeze at high cooling rates. The coating is built up on the substrate by traversing the gun apparatus repeatedly over the substrate building up layer by layer until the desired thickness of coating has been achieved. Because the particles solidify as splats, the resultant microstructure is very lamellar with the grains approximating circular platelets randomly stacked above the plane of the substrate.
A continuing need exists for improvements, however, for improved materials in many fields using thermal spraying.
The present invention relates to the fabrication and use of resistive heaters with a controlled resistance such that when a voltage is applied to the material, heat is generated.
A heater of this invention includes a substrate and a resistive heating layer coated on the substrate. The resistive heating layer includes an electrically-insulating composition mixed with an electrically-conducting composition and a third composition. The electrically-insulating composition has a higher electrical resistance than the electrically-conducting composition.
In a preferred embodiment, the resistive heating layer has the distinctive microstructure of a thermal-sprayed coating and is electrically coupled to a voltage source. In a further preferred embodiment, the third material includes additive that enhances a material property of the resistive heating layer. In another preferred embodiment, the substrate, the electrically-insulating composition and the electrically-conducting composition are ceramic. In yet another preferred embodiment, the third material includes a conductive material or a thermal conductor. In other preferred embodiments, the heater includes a nozzle or a tubular body that heats material within the body.
In a method of this invention, an electrically-conducting composition is mixed with an electrically-insulating composition and a third material to form a mixture. The mixture is then thermal sprayed onto a substrate, and a heater is formed with the sprayed mixture.
In a preferred embodiment of this method, a bonding layer is formed between the substrate and the heater. In another preferred embodiment the substrate is removed from the thermal sprayed mixture. In yet another preferred embodiment, the third material includes a second electrical insulating layer. Still another preferred embodiment includes the formation of a heat reflecting layer. In a further preferred embodiment, a rapid thermal heater for a semiconductor processing system is formed. Additionally, a resistive heating layer is preferably formed that includes silicon carbide, molybdenum disilicide, lanthanum chromate, zirconium diboride or titanium diboride. In the thermal spraying step, a plurality of layers are preferably sprayed.
Another embodiment of this invention is a plasma-sprayed resistive heater including a resistive heating layer including a first electrically-insulating material mixed with a second electrically-conducting material which has a lower electrical resistance than the electrically-insulating material and further mixed with a third material. A voltage source is electrically coupled to the resistive heating layer. Preferably, the electrically-insulating material and the electrically conducting material are both ceramic.
Advantages of embodiments of this invention that have a resistive heating layer include the capability of quickly cycling between processing temperatures due to the extremely low mass of the resistive heating layer. Further, the design of the heater module affords extensive flexibility in heater configuration allowing the heater to be easily designed to include multiple zones of the resistive heating layer that can be independently powered to compensate for unequal heat losses between the center and edge of an article to be heated. Enhanced control over the composition of the resistive heating layer, in accordance with this invention, further provides greater latitude in heater design and greater control over the amount of heat generated.